add excerpt support and fix some bugs

1. fixed #5, new tag <!--more--> to show excerpt in list/timeline, thx
@longlongji
2. fixed #6, fix a bug with can not delete post on BAE, thx @ZoeyYoung
3. fixed #7, fix a bug with tag/post relationship not update when delete
a post
4. add function to clear database memory cache
5. some css fixes